1. Field of the Invention
The present invention relates to an inter-gate insulating film of a memory cell having a stack gate structure.
2. Description of the Related Art
As a structure of a memory cell of a nonvolatile semiconductor memory, there is known a structure in which a floating gate electrode and a control gate electrode are stacked on a gate insulating film. Such a structure is called a stack gate structure (for example, see Jpn. Pat. Appln. KOKAI Publication No. 2002-184875).
Data is written in a memory cell of the stack gate structure in such a manner that electric charges are injected into a floating gate electrode to increase a threshold voltage of the memory cell to a predetermined value. Data is erased on the memory cell in such a manner that electric charges are extracted from the floating gate electrode to decrease the threshold voltage of the memory cell to a predetermined value.
For example, when a NAND flash memory is used as an example, in writing, a write potential of 17V or more is applied to a control gate electrode while keeping a channel of a memory cell (FET) at 0V, and an FN tunnel current is caused to flow from the floating gate electrode to the channel.
In erasing, an erase potential of 16V or more is applied to a semiconductor substrate while keeping the control gate electrode at 0V, and an FN tunnel current is caused to flow from the channel of the memory cell to the floating gate electrode.
In this case, in order to improve writing/erasing efficiency, a coupling ratio of the memory cell must be improved.
In recent years, in order to improve the coupling ratio, as a material of an inter-gate insulating film arranged between a floating gate electrode and a control gate electrode, a material having a higher dielectric constant than silicon oxide (SiO2), such as ONO (oxide/nitride/oxide) or a high dielectric constant (High-k) material is used as a mainstream.
The coupling ratio can be improved by thinning the inter-gate insulating film.
However, when micropatterning of a memory cell is advanced, the above means causes a problem of an increase in leakage current generated in the inter-gate insulating film in writing/erasing, a problem of dielectric breakdown of the inter-gate insulating film, and the like.